Viruses, Viroids, and Prions: Structure, Classification, and Replication

Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are obligate intracellular parasites that require living host cells to multiply. They possess unique features distinguishing them from other microorganisms.

• Nucleic Acid: Contain either DNA or RNA, never both.

• Protein Coat: Surrounded by a protein capsid, sometimes enclosed by an envelope of lipids, proteins, and carbohydrates.

• Replication: Multiply inside living cells using the host's biosynthetic machinery.

• Enzymes: Possess few or no metabolic enzymes of their own.

Host Range

The host range is the spectrum of host cells a virus can infect. Most viruses infect specific cell types within a single host species, determined by:

• Specific attachment requirements to host cell receptors

• Availability of cellular factors required for viral multiplication

Viral Size

Viral sizes vary widely, typically ranging from 20 to 1000 nm. Electron microscopy is required to visualize them. Viruses are much smaller than bacteria and human cells.

Nucleic Acid

Viruses contain either DNA or RNA, which may be single-stranded (ss) or double-stranded (ds), and can be linear or circular. The type of nucleic acid is a key classification feature:

• dsDNA viruses

• ssDNA viruses

• dsRNA viruses

• ssRNA viruses

Capsid and Envelope

The capsid is a protein coat protecting the viral nucleic acid, composed of subunits called capsomeres. Some viruses have an additional envelope derived from the host cell membrane, containing viral and host components. Spikes on the envelope aid in host cell attachment and identification.

General Morphology

Viruses are classified by capsid architecture into several morphological types:

• Helical viruses: Rod-shaped, rigid or flexible

• Polyhedral viruses: Icosahedral shape

• Enveloped viruses: Spherical, with a lipid envelope

• Complex viruses: Complicated structures, e.g., bacteriophages

Taxonomy of Viruses

Viruses are grouped into families based on genomics and structure. Naming conventions include:

• Genus: Suffix -virus

• Family: Suffix -viridae

• Order: Suffix -ales

Viral species share genetic information and ecological niche, often designated by common names (e.g., human immunodeficiency virus).

Viral Multiplication

Viruses must invade host cells and commandeer their metabolic machinery to replicate. Viral genomes encode structural proteins and a few enzymes for nucleic acid processing. A single virion can produce thousands of progeny in one host cell.

Multiplication of Bacteriophages

Bacteriophages (viruses that infect bacteria) can multiply via two mechanisms:

• Lytic cycle: Ends with lysis and death of the host cell.

• Lysogenic cycle: Host cell remains alive, viral DNA integrates into host genome.

Lytic Cycle (T-even Bacteriophages)

The lytic cycle consists of five stages:

1. Attachment: Virus attaches to bacterial cell.

2. Penetration: Viral DNA injected into bacterium; phage lysozyme weakens cell wall.

3. Biosynthesis: Viral nucleic acid and proteins synthesized; host DNA degraded.

4. Maturation: Assembly of viral components into complete virions.

5. Release: Host cell lyses, releasing new phages.

Lysogenic Cycle (Bacteriophage Lambda)

In the lysogenic cycle, viral DNA integrates into the host chromosome as a prophage. The host reproduces normally, copying the prophage. Occasionally, the prophage may excise and enter the lytic cycle.

Results of Lysogeny

• Immunity: Lysogenic cells are immune to reinfection by the same phage.

• Phage conversion: Host cell may acquire new properties (e.g., toxin production).

• Specialized transduction: Certain bacterial genes transferred to another bacterium via phage.

Multiplication of Animal Viruses

Animal viruses follow similar steps as bacteriophages but with differences in entry and uncoating:

• Attachment: Viral attachment sites bind to host cell receptors (proteins/glycoproteins).

• Entry: Via receptor-mediated endocytosis or fusion (for enveloped viruses).

• Uncoating: Separation of viral nucleic acid from capsid inside the cell.

Biosynthesis of DNA Viruses

DNA viruses replicate their DNA in the host nucleus and synthesize proteins in the cytoplasm. Virions are assembled in the nucleus and transported for release.

Biosynthesis of RNA Viruses

RNA viruses use various mechanisms for mRNA formation and genome replication, depending on the type of RNA (positive-sense, negative-sense, or double-stranded).

Retroviridae (Retroviruses)

Retroviruses (e.g., HIV) carry reverse transcriptase, which synthesizes complementary DNA from viral RNA. The resulting DNA integrates into the host genome as a provirus, which may remain latent or be expressed to produce new viruses.

Maturation and Release

• Maturation: Assembly of capsid and viral components.

• Release: Enveloped viruses bud from the host cell, acquiring an envelope; nonenveloped viruses are released by cell lysis.

Viruses and Cancer

Some viruses are oncogenic and can induce tumors by integrating into host DNA and activating oncogenes. About 10% of cancers are virus-induced. Oncogenic viruses include certain DNA and RNA viruses.

Latent and Persistent Viral Infections

Latent infections involve viruses remaining dormant in host cells, sometimes for years (e.g., herpesviruses). Persistent (chronic) infections progress slowly and are often fatal (e.g., HIV, hepatitis B).

Disease	Primary Effect	Causative Virus
Cold sores	No symptoms during latency; virus not usually released	Herpes simplex virus 1
Leukemia	Virus continuously released	HTLV-1, HTLV-2
Cervical cancer	Virus continuously released	Human papillomavirus
HIV/AIDS	Virus continuously released	HIV-1, HIV-2
Measles (subacute sclerosing panencephalitis)	Virus continuously released	Measles virus

Prions

Prions are infectious proteins causing neurological diseases called spongiform encephalopathies (e.g., Creutzfeldt-Jakob disease, kuru). Disease results from conversion of normal prion protein (PrPc) to the infectious form (PrPsc), which accumulates in neural tissue and is always fatal.

Plant Viruses and Viroids

Plant viruses resemble animal viruses in structure and nucleic acid type, causing many crop diseases. Viroids are infectious agents consisting of short, naked RNA molecules, responsible for some plant diseases.